Materials needed by living things are not used up. They are passed around again and again, so we say they cycle through an ecosystem. They move between the abiotic components, which are the non-living parts such as air, water, rocks and soil, and the biotic components, which are the living organisms.
Carbon is a good example. Carbon is found in the air as carbon dioxide. Plants take in carbon dioxide during photosynthesis and use it to make sugars and other compounds, so the carbon becomes part of the plant. Animals get carbon by eating plants or other animals.
Nitrogen is another example. Air is about 78% nitrogen gas, which is abiotic. Plants cannot use nitrogen gas directly, but they take up nitrates from the soil through their roots and use them to make proteins. Animals get their nitrogen by eating.
Water also cycles, and so do other materials such as phosphorus. Because these materials are recycled, the same atoms can be used by many different organisms over time.